The present invention relates to a device for combining light of different wavelengths. The invention relates in particular to an illumination unit with the capability of combining light from red, green and blue narrowband light sources into white light. However, the invention also relates to an illumination unit with the capability of splitting white light into red, green and blue subbeams.
Projectors currently in use which build on the projection of light for image generation can essentially be divided into 2 categories: those which provide for each of the three color channels red (R), green (G) and blue (B) with one imaging element each (3P Projectors=3 Panel Projectors). To the red color channel is assigned light with wavelengths within the wavelength interval of 600 nm to 780 nm. To the green color channel is assigned light with wavelengths within the wavelength interval of 500 nm to 600 nm. To the blue color channel is assigned light with wavelengths within the wavelength interval of 420 nm to 500 nm.
However, there are also those projectors which work with only one imaging element and operate color sequentially (CS Projectors=Color Sequential Projectors).
A further classification can be based on the manner in which the imaging element modulates light in order to pass on image information. A widely established class of image producing elements subjects the incident light to a locally resolved polarization modulation. This polarization modulation is subsequently transferred into an intensity modulation by means of polarization-selective optical elements. This type of imaging element must be impinged by polarized light. However, the focus of the present description are illumination devices for another class of imaging elements, which can be impinged by nonpolarized light or by light only partially polarized. The illumination devices required for this purpose should have the capability of preparing nonpolarized light for impingement. If broadband white light sources are utilized in 3P projectors, the white light must first be split into the three colors red, green and blue. One possibility of carrying this out is the utilization of dielectric edge filters. An edge filter has the task of reflecting nearly 100% of light in a first wavelength range, while it should transmit nearly 100% of the light in a second adjacent wavelength range. The region in which the wavelength ranges adjoin is denoted as a filter edge. If a first edge filter with a filter edge at 500 nm is placed into the beam path of a white light source, the blue light assigned to the blue color channel is first split from the yellow light. Yellow light in this case is additively combined green and red light. If an edge filter with an edge at 600 nm is placed into the beam path of the yellow light, green light is split from red light.
The implementation of the particular edge filter determines which wavelength range is reflected or transmitted. An edge filter which transmits the wavelength range with the shorter wavelengths, while the longer wavelengths are reflected, is generally referred to as shortpass filter. An edge filter which reflects the wavelength range with the shorter wavelengths while transmitting the longer wavelengths, is referred to as longpass filter.
If narrowband light sources, such as for example the light from LEDs, are utilized in CS projectors, the illumination configuration has the task of joining the light paths of a red, green and blue narrowband light source and to direct the light beams onto the one imaging element. Edge filters can again be employed: a first filter which, for example, combines the light path of the red and of the green light and a second one, which combines the light path of the blue light with the two other light paths.
A problematic aspect is the fact that light from white light sources as well as also light of narrowband LEDs, as a rule, does not supply polarized light, but in any event incompletely polarized light.
Edge filters are, however, typically realized by means of dielectric interference layer systems on glass substrates which are otherwise transparent. Interference layer systems with respect to polarization dependency, however, have characteristics which, in the case of the edge filters described here, have been found to be disadvantageous. In order not to reflect a portion of the light back into itself, the edge filters are disposed at an angle which is inclined with respect to the optic axis. It is problematic here that because of this the reflection and transmission behavior of the interference filter becomes polarization dependent. In particular the position of the edge as well as also the reflection and transmission in the wavelength ranges adjacent to the edge depend on the polarization. In light sources operating with non-polarized or only partially polarized light, this leads to erroneous misdirection of light components. For one, this leads to a loss of light and, for another, can have an unfavorable effect on the particular color coordinates.
In the present specification the optical path which must be traversed by the blue component of the light is referred to as the blue channel. The proportion of the blue light emitted by the light source arriving at the imaging element, is referred to as the blue channel transmission. A red channel transmission and a green channel transmission are referred to correspondingly. It is understood that misdirections of light components lead to a decrease of the channel transmission.
A further important effect influencing the channel transmission is the angular emission characteristic of the light source or of the light sources. The optical elements and filters utilized for the illumination must therefore have a certain angular acceptance, which, as a rule, is expressed as the f-number. The f-number is inversely proportional to the numeric aperture (NA) defined by the product of the index of refraction of the medium and one half of the aperture angle of the illumination cone, i.e. the smaller the f-number, the greater the required angular acceptance. The effect exerted by the different angles of incidence onto the transmission characteristic of the edge filters, must also be taken into consideration in calculating the channel transmission. The position of the edge as well as also the reflection and transmission in the ranges adjoining the edge depend on the angle of incidence. In order to take this into consideration, weighted integration is carried out over the different angles of incidence. For the channel transmission this means that the initially steep edges for an angle of incidence through integration over different angles forfeit steepness and consequently, light in the edge region is misdirected.